Micro-machined Vibrating Structure Gyroscopes (VSG) are widely available at low cost. Such gyroscopes have been fabricated using a wide range of vibrating structures which include tuning forks, planar rings, beams and oscillating disk structures. The basic operating principles of all these gyroscopes are essentially the same in that the vibrating structure is driven into resonance to provide a carrier mode motion. When the structure is rotated around an axis orthogonal to the linear motion provided by the carrier mode motion, Coriolis forces are developed. These forces are directed along the remaining orthogonal axis and cause the vibrating structure to vibrate in a second mode of oscillation called the response mode. The motion of this response mode is in phase with the carrier mode motion with the amplitude being directly proportional to the applied rotation rate.
Such Vibrating Structure Gyroscopes are suitable for use in a wide range of high volume applications such as in the automotive field for automatic braking systems, roll over prevention and car navigation. The low cost and small size of these Vibrating Structure Gyroscopes makes them attractive for other uses such as inertial navigation and platform stabilisation. However their use in the latter applications has been limited by the restricted performance that can be achieved, particularly in terms of bias stability (output in the absence of an applied rate). It is necessary to achieve improved performance from these Vibrating Structure Gyroscopes to make them suitable for applications requiring greater accuracy.
A major limitation which restricts the performance of vibrating structure Coriolis gyroscopes is quadrature bias error. Quadrature bias errors arise due to the imperfections in the geometry of the vibrating structure. These imperfections cause oscillation of the response mode which is in phase quadrature (i.e. has a 90° phase relationship) to the motion induced by applied rotation rates, and may be present even when the gyroscope is not rotating. The magnitude of these signals may also be large in comparison to the required in-phase signal which provides the rotation rate information. Recovering the required rotation induced signal in the presence of a large quadrature signal places stringent requirements on the phase accuracy of the detection system. Accurately phased electronics enable the quadrature signal to be substantially rejected. However practical limitations on the accuracy with which this phasing can be set mean that some of this signal will typically remain to contaminate the true rotation induced in-phase signal. This limitation is a major source of error for this type of Vibrating Structure Gyroscope.
There is therefore a need for a method for further minimising the impact of quadrature error on vibrating structure Coriolis gyroscope performance.